Image registration for production of four color images from separations has long been a problem. In order
directly on the photoconductor, it is necessary to image to expose the photoconductor through the toner after the first toner has been deposited. This necessitates that the subsequently applied toners be transparent to the exposing radiation. For reasons of practicality and economics, a single source of radiation at a single wavelength is used (e.g., a laser). However, the acceptable colorants for four color half-tone processes (particularly black toners) have no common spectral region of low absorbance (i.e., high transparency) in the range of 400-750 nm.
Solutions to this problem have heretofore been unsatisfactory. They have included exposing the backside of the photoconductor through a transparent support; the use of several wavelengths of light and the placement of the black toner last; and the transfer of the toner after each deposition. Toners which have been suggested for these solutions are known.
U.S. Pat. No. 4,145,299 discloses an electrographic liquid developer comprising a carrier and marking particles which are formed by coupling diazonium salts with 2,3-naphthalenediol derivatives. These particles are said to absorb radiation relatively uniformly in the range of 400 to 700 nanometers (nm) and exhibit neutral density coloration, that is they are black or nearly black in hue. However, these toners have not been found to be stable to extended ultraviolet or visible radiation.
U.S. Pat. No. 4,414,152 discloses bis aryl-azo compounds having a base structure similar to those of U.S. Pat. No. 4,145,299. These compounds are said to be useful as neutral density pigments in electrophotographic developers.
U.S. Pat. No. 4,654,282 discloses a method of forming a toner image by overlapping one or more previously formed toner images. Liquid developers are disclosed as being useful in the invention. Colorants which can be used in these developers include toners of the type described in U.S. Pat. No. 4,145,299 and U.S. Pat. No. 4,414,152.
The foregoing solutions have not been entirely satisfactory. They have not provided a process in which the photoconductor is exposed through the toners so that all colors can be laid down in any order then transferred at one time.